F and E tank keeps filling with hot water

[theglovebox]

I have an issue with the fill and expansion tank in my loft. I have a conventional open vented system. Recently the tank was overfilling and the overflow pipe was not sealed properly so we had water leak through the ceiling. After some investigation I found the tank was filling with hot water from the expansion pipe (not the cold water feed). I have plumbing insurance and a plumber came out to fix. He could not answer why the tank kept filling but suggested fitting a new F and E tank and “see if it settles down”.
The tank still intermittently fills with hot water(the same as before), but strangely now the level never seems to go up and does not come near the overflow. I don’t think this is normal and have read lots of threads with similar, but not identical issues. For what it’s worth:
I have bled any air from the radiators
The heating is not being used currently
The hot water seems to be a normal colour
I cannot see any leaks in the system

My suspicion is there is a pin hole in the copper coil in the hot water cylinder, but the water is a normal colour and I can’t figure where the overfilling water is going. Does anyone have any thoughts or advice? I suspect I will get a plumber out again but hopefully one who can solve the underlying issue. Thanks in advance

PS - thanks to those who replied to this post that was accidentally posted in the wrong forum

 

[terryplumb]

Is water still coming from the vent pipe over the f&e tank ? If so the system is pumping over.

 

[Johntheo5]

Is the feed and expansion tank higher than the cold feed supply tank?.
Did you get a new circ pump installed recently? Whether or not, what is its make/model and what mode & speed is it set to?

 

[theglovebox]

Is water still coming from the vent pipe over the f&e tank ? If so the system is pumping over.

Yes, it is, but the tank is not overfilling. I don't understand where the water is going??

 

[theglovebox]

Is the feed and expansion tank higher than the cold feed supply tank?.
Did you get a new circ pump installed recently? Whether or not, what is its make/model and what mode & speed is it set to?

Thanks. The cold water tank and F and E tank are at the same level, the water in the cold water tank is obviously higher than that in the F and E tank. The pump has been the same one for years. It is a Terrier TC5, it seems to be set to level 3 (of 3) but judging by the dust on it has not been changed in a long time.

 

[terryplumb]

,

Yes, it is, but the tank is not overfilling. I don't understand where the water is going??

It's going back into the system , and pumped back into the tank ,in a "loop " for want of a better description !
Set the pump to 1, see if it makes a difference.

 

[Johntheo5]

As above, try speed 1 or 2, get someone to start/stop the circ pump and watch for spurts of water from the vent pipe, you may have a restriction due to sludge build up/blockage especially where the cold feed joins the system, post a schematic if possible, you may have the pump on theboilerflow where youhave the boiler then the vent then the cold feed and then the pump, VCP system.

 

[Notch7]

I have heard sludge build up in critical parts of the system can cause this.